The present invention relates to a room temperature vulcanizable silicone rubber composition and more particularly the present invention relates to a room temperature vulcanizable silicone rubber composition which has a low vapor transmission rate as the result of having incorporated into the composition a mica filler.
Room temperature vulcanizable silicone rubber compositions are well known. Particularly, the two component or two part room temperature vulcanizable silicone rubber compositions. Such compositions generally comprise a silanol end-stopped diorganopolysiloxane as the base polymer, a reinforcing filler selected from precipitated silica and fumed silica and optionally an extending filler such as zinc oxide, iron oxide, diatomaceous earth, quartz, ground quartz, etc. Normally the silanol end-stopped diorganopolysiloxane is mixed with the filler and is packaged separately. In the other package or component there is provided a cross-linking agent which can be an alkyl silicate or a partial hydrolysis products of such silicates. Optionally as an alternative cross-linking agent there can be present in the composition a cross-linking agent, a silicone resin composed of monofunctional units and tetrafunctional units. There must be also present in the composition a metal salt of carboxylic acid as a catalyst to speed up the rate of reaction of the composition to form a silicone elastomer. Usually the cross-linking agent and the catalyst are packaged in a separate package or component. Either package can have additional ingredients as long as the cross-linking agent and the catalyst are kept separate from the silanol end-stopped diorganopolysiloxane polymer. When it is desired to cure the composition the two components are mixed together to form a homogeneous mixture which then cures in a period of time varying anywhere from a half hour to 24 hours to obtain the desired silicone elastomer. Another two-part room temperature vulcanizable silicone rubber composition that is also well known in the SiH Olefin platinum catalyzed compositions in which a vinyl containing polysiloxane polymer is packaged separately with a filler and in which the second part of the composition comprises a hydride silane or siloxane packaged with platinum. The platinum in this case can be present in either the hydride cross-linking agent or with the vinyl containing polymer, as long as there is not, in any one package or component a combination of the platinum with the hydride cross-linking agent with some of the vinyl containing polymer. Accordingly in such SiH Olefin platinum catalyzed compositions the two components are mixed together and either at room temperature or at elevated temperatures cured to form a silicone elastomer. Such two component room temperature vulcanizable silicone rubber compositions are well known as encapsulant sealants. Normally such two component room temperature vulcanizable silicone rubber compositions accomplish their sealant function especially for the sealing of glass panes and partitions to masonry and other types of base structure material with desirable efficiency. However, they have one disadvantage in that as previously formulated they have a high moisture vapor transmission rate. Such prior art two-component room temperature vulcanizable silicone rubber compositions have vapor transmission rate of somewhere in the are of 40 to 50 grams per square meter in a 70 mils. thick section of silicone elastomer. As stated previously for most application this did not create a problem. However, the high vapor transmission rate was exceedingly undesirable in the case the silicone material was used to enclose a sealed area in which vapor accumulation was a problem. The need for such low vapor transmission rate for silicone sealants was even more noticeable in the case where it was desired to fabricate insulated window panes so that the glass would be insulated by the air pocket between the panes. Thermal panes are the best glass insulated glass partitions known in the industry. To produce thermo panes two glass sheets are hermitically sealed to each other with a layer of air in between the panes which is completely dry such that moisture cannot permeate and enter the air barrier between the glass panes. The manufacture of such thermal panes is expensive which makes them unduly costly for insulation in high rise buildings. Accordingly, in a recent innovation in the industry a facsimile of thermo panes is produced by taking two clear glass panes and sealing the edges with a sealant such as a polysulfide sealant, which polysulfide sealant is again applied to the sealed glass pane so as to seal the insulated window pane to the frame of the building. Such polysulfide sealants have a moisture vapor transmission rate of about 15 grams per meter square of 70 mils thick sealant. Although suitable to some extent the polysulfide vapor transmission rate as noted above is still a little too high. Accordingly, in some cases when such fabricated insulated glass panes are tested by cycling them from a low temperature of -60.degree. F to 150.degree. F there is invariably too much moisture that has migrated through the sealant into the air space between the glass panes and as a result the glass panes will fog up with water vapor. As an improvement there has been inserted between the two panes of the insulated glass construction a butyl rubber tape having a dessicant therein over which there is applied the polysulfide sealant so as to maintain a low vapor transmission rate into the air space between the glass panes. The use of silica gel or micro seives or another dessicant with the butyl rubber tape and the polysulfide sealant results in a combination that had a low moisture transmission rate. However, such construction of insulated glass panes is very expensive. Furthermore, the construction utilizing the butyl tape with the dessicant therein is still undesirable in that polysulfides do not have as good weatherability as is experienced with silicones. Further, as can be envisioned there could be utilized a butyl rubber tape between the two glass panes with prior art silicone rubber compositions. However, in some cases too much moisture is transmitted into the air pocket between the glass panes. In addition, the silicone butyl rubber tape combination cannot be utilized for sealing the insulated glass panes to the frame of high rise buildings. Accordingly, it would be highly desirable to have a low vapor transmission rate silicone sealant.
It is an object of the present invention to provide for a two component low moisture vapor transmission rate room temperature vulcanizable silicone rubber composition in which the cross-linking agent in the composition is either a silanol terminated resin composed of monofunctional or tetrafunctional units or a silicate.
It is yet an additional object of the present invention to provide for two component room temperature or heat vulcanizable silicone rubber composition with a low vapor transmission rate where the composition cures to an elastomer by the utilization of an SiH olefin platinum catalyzed reaction. It is yet an additional object of the present invention to provide for a process for forming either a heat vulcanizable or room temperature vulcanizable two component silicone rubber compositions with a low vapor transmission rate wherein the silicone elastomer is formed in an SiH olefin platinum catalyzed reaction.
These and other objects of the present invention are accomplished by the means of objects set forth herein below.